Abstract Rio Blanco-Los Bronces, one of three giant late Miocene to early Pliocene copper deposits in the Andes of central Chile, formed as a result of emplacement of both multiple mineralized breccias and porphyry intrusions into early and middle Miocene plutonic rocks and Cenozoic lavas. The deposit is distinctive in that a significant proportion of the >50x10 6 metric tons of Cu it contains occurs within the matrix of mineralized breccia pipes or disseminated in the host rocks around the breccias. Approximately 50 percent of the Cu ore in the deposit occurs as breccia-matrix, stockwork, and disseminated mineralization in a zone of potassic alteration which formed during the emplacement of biotite-rich breccias of the Rio Blanco breccia complex and quartz monzonite porphyry intrusions. Following a period of uplift and erosion, younger tourmaline-rich breccia pipes, containing the other 50 percent of the Cu in the deposit, and weakly mineralized early Pliocene porphyries were emplaced both within and peripherally to the earlier zone of biotite breccias and potassic alteration. Clasts within the tourmaline breccias are sericitized. This sericitic alteration developed during the emplacement of these breccias, later than and independent of the earlier potassic alteration. Fluid inclusion and 0-, S-, and H-isotopic data indicate that the metal-rich fluids that generated both the older biotite-rich and younger tourmaline-rich breccias, and caused the potassic and sericitic alteration associated with these two breccia generations, were magmatic in origin. Sr- and Nd-isotopic data imply that the magmas that exsolved the breccia-forming fluids, as well as those that formed the late porphyries, were distinct isotopically from the older host rocks of the deposit. The breccia-forming fluids are believed to have exsolved from magmas that crystallized to form plutons that are still not exposed at the surface, consistent with the deep, as yet unknown roots of these breccias. The emplacement of the mineralized breccias and porphyries at Rio Blanco-Los Bronces occurred in conjunction with late Miocene changes in Andean magma chemistry and crustal thickness, within a period of<3 m.y. during the final stages of>15 m.y. ofMiocene magmatic activity in central Chile. The temporal changes in magma chemistry, the crustal thickening, uplift, and erosion which caused the younger mineralized tourmaline breccias to be superimposed on the earlier and deeper potassic alteration zone, and the decline of igneous activity in the Miocene magmatic belt all resulted from decreasing subduction angle beneath central Chile beginning in the middle to late Miocene.

The mean remanent magnetization vector of six samples from andesite dikes at one locality after 15 mT ODF is D = 309, I = +77, k = 13, α 95 = 20. Five rhyolite samples from each of two sites at a second locality yield: 50 mT ODF, D = 194,1 = +30, k = 166, α 95 = 6; 30 mT ODF D = 158,1 = +46, k = 101, α 95 = 8. These rocks are probably of Mesozoic age and the magnetization appears primary; but the vectors are unlike those for the stable South American craton at any time during the Phanerozoic. These data suggest that complex tectonic rotations have occurred in at least this part of the Santander Massif.

A total of 56 fission-track ages are reported on 21 apatite, 19 zircon and 2 sphene concentrates from 24 rock samples collected from Toas Island (3) and the Sierra de Perijá (9), Western Venezuela, and from the Santander Massif (12), Eastern Colombia. All apatite ages, set by cooling on uplift, date points on uplift curves. Portions of such uplift curves are late Oligocene (27–22 Ma) in the southeast piedmont of the Sierra de Perijá; early to middle Miocene (19–14 Ma) in the western piedmont and (16–14 Ma) in the central Santander Massif; middle Miocene (13 Ma) in Toas; late Miocene to early Pliocene (7–4 Ma) in the central and northern Santander Massif; and middle Pliocene (3 Ma) in the Sierra de Perijá. When apatite data for the Venezuelan Andes are added to the above and integrated with regional geologic evidence, it is concluded that Tertiary uplift became progressively greater and occurred at faster rates but through shorter time spans. On an elevation/age plot this should be revealed as a fan-shaped array of uplift curves. Because of inadequate topographic relief at most localities, this cannot be proven but is suggested as a working hypothesis for future verification. Zircon ages reported here range from about 50 to 126 Ma. When reviewed together with zircon ages from the Venezuelan Andes, it is considered that 8 or 9 ages from the Venezuelan Andes (3), Santander Massif (3), and Perijás (2 or 3) reflect uplift in end- Cretaceous-Paleocene time. Two dates from Toas and the Perijás may give original crystallization ages of felsic volcanics (120–122 Ma). The remaining 23 ages are interpreted as mixed ages related to partial annealing of clocks set in Permian–early Cretaceous time although a pronounced concentration in the range of 85 to 101 Ma raises the possibility of some regional tectono-thermal event at that time. A satisfactory explanation of the latter remains to be found. The regions referred to above are among the principal tectonic elements of a triangular continental block with apex in Santa Marta, Colombia, and the Oca and Santa Marta–Bucaramanga faults as sides. In this region “Andean” uplift is considered to have been initiated in end-Cretaceous-Paleocene time in response to northwest-southeast compression affecting the Caribbean and South American plates. Progressive compression in the apical direction of the triangle resulted in progressive interlocking of crustal blocks and final major uplift in unison during the Pliocene-Pleistocene(?). The two boundary faults are interpreted as high-angle, oblique-slip type with modest strike-slip component of movement.